Re-add `mouse_focus`-nullcheck #88

Job	Run time
📊 Static checks / Code style, file formatting, and docs	3m 14s
🍏 iOS / Template (target=template_release)	30m 54s
🐧 Linux / Editor w/ Mono (target=editor)	1h 7m 17s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	1h 9m 4s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	56m 1s
🐧 Linux / Template w/ Mono (target=template_release)	40m 29s
🐧 Linux / Minimal template (target=template_release, everything disabled)	20m 17s
🍎 macOS / Editor (target=editor, tests=yes)	45m 11s
🍎 macOS / Template (target=template_release)	35m 44s
🏁 Windows / Editor (target=editor, tests=yes)	1h 24m 1s
🏁 Windows / Template (target=template_release)	56m 58s
🌐 Web / Template (target=template_release)	22m 31s
🤖 Android / Template (target=template_release)	43m 6s
	9h 34m 47s

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